You've pretty much eliminated the belts, although you could check pulley alignment just to be sure. That pretty much leaves bad bearings in one of the accessories -either alt, p/s, a/c or water pump. Your description smells more like a complaining water pump to me. Nothing overly fatal, the pump will continue to get louder in the higher rpms and eventually you'll start seeing rad fluid dripping out. Are you losing rad fluid lately? Just avoid letting the temp gauge needle stay in the red for any length of time. You may (and I emphasize "may") get a little extra life out of the water pump by adding a bottle of water pump lubricant to the rad fluid.